Incandescent light sources are low cost but have low efficiency, and are relatively large requiring large light fittings. Fluorescent lamps in which a gas discharge generates ultraviolet wavelengths which pumps a fluorescent material to produce visible wavelengths, have improved efficiency compared to incandescent sources, but also have a large physical size. Heat generated by inefficiencies in these lamps is typically radiated into the illuminated environment, such that there is typically little need for additional heatsinking arrangements. In this specification, an illumination apparatus refers to an illumination apparatus whose primary purpose is illumination of an environment such as a room or street scene, or as a display backlight such as an LCD backlight. An illumination apparatus is typically capable of significantly higher luminance than 1000 nits. This is opposed to for example displays, whose primary purpose is image display by providing light to a viewing observer's eyes so that an image can be seen. By way of comparison, if the luminance of a display is very high, for example greater than 1000 nits, then disadvantageously a display can be uncomfortably bright to view. Thus the considerations for an illumination apparatus with a primary illumination purpose and a display apparatus that provides a secondary illumination purpose are different.
If an illumination apparatus is used as a backlight in a display apparatus, losses in the spatial light modulator of the display apparatus will reduce the luminance to a level suitable for comfortable viewing. Thus such an arrangement has a secondary illumination function that is not generally suitable for the purpose of efficient and bright illumination of an environment.
Light-emitting diodes (LEDs) formed using semiconductor growth onto monolithic wafers can demonstrate significantly higher levels of efficiency compared to incandescent sources. In this specification LED refers to an unpackaged LED die (chip) extracted directly from a monolithic wafer, i.e. a semiconductor element. This is different from packaged LEDs which have been assembled into a package to facilitate subsequent assembly and may further incorporate optical elements such as a hemispherical structure which increases its size but increases light extraction efficiency. To optimise quantum efficiency, extraction efficiency and lifetime, it is desirable to minimise the junction temperature of the LED. This is typically achieved by positioning a heat dissipating structure (or heatsink) on the rear of the LED to provide extraction of heat from the chip into an ambient environment.
LED primary heatsinks typically comprise heat slugs (or heat spreaders), LED electrodes, and the dielectric layer of a metal core printed circuit board (MCPCB). LED secondary heat sinks typically comprise the metal layer of the MCPCB, MCPCB solder attachment points and formed fins in metal or thermally conductive plastic material attached to or formed on the primary heatsink arrangement. For illustrative purposes, in this specification, primary thermal resistance refers to the thermal resistance to heat generated in a light emitting element formed by the light emitting element itself, respective heat spreading elements, electrodes and electrically insulating support substrate (such as the dielectric layer of an MCPCB). The secondary thermal resistance is defined by the thermal resistance of subsequent elements, including the metal layer of an MCPCB, MCPCB solder attachment points and heatsink elements.
Assembly methods for known macroscopic LEDs typically of size 1×1 mm comprise a pick-and-place assembly of each LED chip onto a conductive heat slug for example silicon. The heat slug is attached to a dielectric which is bonded on a metal layer, forming a metal core printed circuit board (MCPCB). Such a primary heatsink requires multiple pick-and-place operations and is bulky and costly to manufacture. It would thus be desirable to reduce primary heatsink complexity.
Secondary heatsinks can be heavy, bulky and expensive. It is thus desirable to minimise the thickness of the secondary heatsink by minimising the resistance of the thermal paths of the primary heatsink.
In lighting applications, the light from the emitter is typically directed using a luminaire structure to provide the output directionality. The angular variation of intensity is termed the directional distribution which in turn produces a light radiation pattern on surfaces in the illuminated environment and is defined by the particular application. Lambertian emitters provide light to the flood a room. Non-Lambertian, directional light sources use a relatively small source size lamp such as a tungsten halogen type in a reflector and/or reflective tube luminaire, in order to provide a more directed source. Such lamps efficiently use the light by directing it to areas of importance. These lamps also produce higher levels of visual sparkle, in which the small source provides specular reflection artefacts, giving a more attractive illumination environment. Further, such lights have low glare, in which the off-axis intensity is substantially lower than the on-axis intensity so that the lamp does not appear uncomfortably bright when viewed from most positions.
Directional LED illumination apparatuses can use reflective optics (including total internal reflective optics) or more typically catadioptric (or tulip) optic type reflectors, as described for example in U.S. Pat. No. 6,547,423. Catadioptric elements employ both refraction and reflection, which may be total internal reflection or reflection from metallised surfaces.
PCT/GB2009/002340 describes an illumination apparatus and method of manufacture of the same in which an array of microscopic LEDs (of size for example 0.1×0.1 mm) is aligned to an array of micro-optical elements to achieve a thin and efficient directional light source. GB1005309.8 describes an illumination apparatus, a method of manufacture of the same and a heat sink apparatus for use in said illumination apparatus in which an array of optical elements directs light from an array of light emitting elements through a heat dissipating structure to achieve a thin and efficient light source that provides directional illumination with efficient dissipation of generated heat into the illuminated environment.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.